linux-kernel - Re: [PATCH 7/7] Simple Performance Counters: SLUB instrumentation

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Message-ID: <Pine.LNX.4.64.0708201239070.29624@schroedinger.engr.sgi.com>
Date:	Mon, 20 Aug 2007 12:41:36 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 7/7] Simple Performance Counters: SLUB instrumentation

On Fri, 17 Aug 2007, Mathieu Desnoyers wrote:

> > The cpu will definitely not be idle during these measurements and no 
> > frequency scaling is active.
> 
> The problem is that if the cpu is idle _before_ the measurements, the
> frequency will change differently from one cpu to another. Therefore,
> the cycle counters may have large offsets when you start your tests. So,
> if get_cycles() is executed on different CPUs (thread being migrated)
> between the beginning and the end of the test, the results would be
> skewed.

TSC measurements as done by this patch are associated with cpus. The 
skew is irrelevant. Development of this patchset was done on a 
system whose TSC are never synchronized.
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